The present invention relates to electromagnetic apparatus for producing linear motion. One means of producing linear motion directly by electromagnetic means is the linear electric motor devised by Professor Laithwaite of Imperial College, London. This device can be considered as a conventional rotary electric motor in which the stator and armature have been split axially and unrolled. Although the device is very effective, it does have the disadvantage that considerable unbalanced attractive forces exist between the stator and moving armature, so that some means such as a track or wheeled carriage has to be used to maintain a separation between the stator and the armature.
A second form of linear motor has a cylindrical topology like a conventional electric motor but with the magnetic field-producing elements disposed transversely around the longitudinal axis so that the magnetic fields alternate in polarity along the axis of the motor.
Examples of such linear motors are shown in OLS 2,229,711; EP 0 221 228; U.S. Pat. No. 4,714,300; EP 0 280 743; EP 0 314 493 and WO 93/01646.
OLS 2,229,711 discloses a DC motor in the form of a piston with a single coil which moves in a cylinder under the action of magnetic fields which are generated by the successively energizing and de-energizing ring-shaped coils spaced axially along the inner surface of the cylinder surrounding the piston.
EP 0 221 228 discloses an arrangement in which both linear and rotary motion of a shaft are produced, the linear motion of the shaft being produced by circumferential coils and the rotary motion by axial peripheral coils.
U.S. Pat. No. 4,714,300 discloses a pressure modulator for use in anti-skid brake systems in which a plunger is made to oscillate longitudinally in a chamber under the influence ofa longitudinally-varying magnetic field pattern generated between a winding on the inside of an outer portion of the chamber and a number of magnets disposed around the periphery of the plunger, or vice versa.
EP 0 280 743 discloses a screw for an injection molding machine which is moved linearly by the interaction between magnets on a shaft forming part of the injection screw and a number of coils surrounding the shaft and to which a three-phase alternating current is applied.
EP 0 314 493 discloses another arrangement in which a shuttle is caused to oscillate in a chamber by the interaction between two magnetic fields, one generated by a winding on the shuttle and the other by a winding forming part of the chamber wall.
WO 93/01646 discloses another arrangement in which a piston is driven longitudinally in a cylinder by means of the interaction between two radial magnetic fields, one generated by a series of circumferential coils forming part of the cylinder wall and the other by a plurality of magnets forming part of the piston, or vice versa. For many purposes, the arrangements disclosed in WO 93/01646 are quite satisfactory. However, if the invention of WO 93/01646 is to be used as a combined fluid/electromagnetic actuator, the need to provide fluid-tight seals both on the piston and where the piston rod emerges through an end wall of the cylinder is a disadvantage, particularly as heavy and off-axis loads will tend to bend the piston rod out of true. Also, the solid construction of the piston and piston rod means that larger sizes of the device will become very heavy.